INFLUENCE OF CERTAIN ELEMENTS ON STEEL. 351
under shock, so that the permissible maximum has been gradually lowered, and the standard product of the present day contains from .70 to 1 per cent. In higher steels the same lesson has. been learned, but in this case the necessity of a low content is far more marked, since a percentage which is perfectly harmless in un-hardened steel will cause cracking if the metal be quenched in water.
In structural metal there is no quenching to be done and the line of maximum manganese need not be drawn too low. It is more convenient to produce a higher tensile strength by the use of spiegel-iron than with ordinary pig-iron, since manganese deadens the metal and prevents the oxidation of the carbon. Thus an increased strength resulting from the addition of more recarburizer is usually accompanied by an increase in the manganese, and it is currently assumed that a considerable part of the extra strength is due to the higher percentage of this element. In great measure this is an error, for the increase in carbon is often sufficient to account for the change.
Ferro-manganese containing 80 per cent, of manganese holds about 5 per cent, of carbon, and since one-third of the manganese is lost during the reaction while very little carbon is burned, it follows that fX80=53 points of manganese will be added to the steel for every 5 points of carbon. Thus, if the content of manganese in any heat be raised .20 per cent, by an increase in the recarburizer, there will at the same time be an increment of .02 per cent, of carbon. This slight change in carbon will not always be detected by the color method, particularly as an increase in manganese interferes with the accuracy of the comparison by altering the tint of the solution, and so the effect of this carbon, representing an increase in. strength of 2400 pounds per square inch, is often ascribed to the increment of manganese. It is necessary, therefore, to compare steels where the composition is thoroughly known, to find the effect of this element.
It is currently believed that manganese reduces the ductility of steel, but Table XVII-E will show that the effect is not well marked. This table is made by grouping heats of the same general character and of about the same strength, and separating them into two classes according to their manganese content. ITo arbitrary line is drawn between a high and low percentage, but each group is divided so